Hitherto, it has been a conventional practice to use a Zener diode suited for a surge absorber. For example, when a Zener diode is reversely connected to the input side of an electric circuit to be protected, the Zener diode serves as a high resistance against a low voltage of usual signals, and the Zener breakdown occurs in the diode under a high voltage applied to the circuit, which leads the overcurrent flow to the Zener diode to protect the circuit or the semiconductor devices used there from breaking down due to the heavy current.
There are two conventional methods for manufacturing a Zener diode; the first one is to employ a semiconductor substrate having impurity concentration corresponding to the aimed Zener breakdown voltage, and the second one is to grow on a semiconductor substrate an epitaxial semiconductor layer having impurity concentration corresponding to the aimed Zener breakdown voltage.
In manufacturing a Zener diode according to the first method, as shown in FIG. 6, impurities such as boron or indium are diffused into an n-type semiconductor substrate 31 having a thickness of about 130 .mu.m and a resistivity of about 5 to 600 m.OMEGA..multidot.cm corresponding to the aimed Zener breakdown voltage to form a p.sup.+ -type diffusion region 32 and a pn junction 33. Further, an insulating film 34 made of silicon oxide or the like is formed on the surface of the semiconductor substrate 31 by a CVD process, followed by the formation of a contact hole 35 in a portion of the insulating film 34 for an upper electrode. An n.sup.+ -type diffusion layer 36 is formed over the entire lower surface of the semiconductor substrate 31 so as to provide an ohmic contact between a lower electrode and the semiconductor substrate 31. Then a metal like aluminum is deposited by, for example, sputtering on the p.sup.+ -type diffusion region 32 on the upper side of the substrate 31 and on the n.sup.+ -type diffusion layer 36 on the lower side respectively to form upper and lower electrodes 37 and 38. Finally, the semiconductor substrate 31 is diced into individual chips to complete a Zener diode 39. This method tends to lower the yield because inconstant resistivity of the semiconductor substrate affects Zener characteristics.
On the other hand, in manufacturing a Zener diode according to the second method, as shown in FIG. 5, an n.sup.+ -type semiconductor substrate 40 is used which has a thickness of 110 .mu.m and a low resistivity of about 1/1000 to 20/1000 .OMEGA..multidot.cm. In view of an ohmic contact with the electrodes, the semiconductor substrate 40 is selected to have a low resistivity with impurities of 10.sup.18 atm/cm.sup.3 or more. On the surface of the semiconductor substrate 40 n-type semiconductor layer 41 is epitaxially grown upto a 20 .mu.m-thick which has a resistivity of about 5 to 600 m.OMEGA..multidot.cm corresponding to a desired Zener breakdown voltage. In a manner similar to the first method a p.sup.+ -type diffusion region 32, insulating film 34 and upper electrode 37 are formed. Finally a metal is deposited directly on the lower surface of the semiconductor substrate 40 to form a lower electrode 38, thus completing a Zener diode 42.
When the reverse voltage is applied to the diode manufactured by either of the first or second method to increase the reverse current, the characteristic curve with respect to voltage V.sub.R and current I.sub.R is shown in FIG. 7. A current begins to flow at the Zener breakdown voltage depending on the diode, and a breakdown occurs when the input reaches the breakdown withstand power (allowable loss point C), that is I.sub.2 .times.V.sub.2, of the diode. Therefore, it is a problem that the diode tends to be broken down under a noise with a large surge current.